Recent developments in digital imaging include the use of linear arrays which operate as one-dimensional spatial light modulators. Images formed using a linear array are generated one line at a time, then scanned over a surface for display or printing applications. Linear arrays have been recognized to have some inherent advantages over two-dimensional liquid crystal displays (LCD) and digital micromirror displays (DMD) with area spatial light modulators, including the capability for higher resolution, reduced cost, and simplified illumination optics. Particularly where a high degree of color saturation, optimized color gamut, and good light intensity are important, linear arrays of electromechanical grating devices are particularly well-suited for use with laser light sources and are recognized to be, in many ways, superior to their two-dimensional counterparts for modulating laser light. For example, Grating Light Valve (GLV) linear arrays, as described in U.S. Pat. No. 5,311,360, issued May 10, 1994, titled “Method And Apparatus For Modulating A Light Beam,” by Bloom et al., are one earlier type of linear array that offers a workable solution for high-brightness imaging using laser sources. U.S. Pat. No. 5,982,553, issued Nov. 9, 1999, titled “Display Device Incorporating One-Dimensional Grating Light-Valve Array,” by Bloom et al. discloses a display apparatus that modulates light using a diffractive linear light valve array of electromechanical grating devices.
Recently, an electromechanical conformal grating device that includes ribbon elements suspended above a substrate by a periodic sequence of intermediate supports was disclosed by Kowarz in U.S. Pat. No. 6,307,663, issued Oct. 23, 2001, titled “Spatial Light Modulator With Conformal Grating Device.” The electromechanical conformal grating device is operated by electrostatic actuation, which causes the ribbon elements to conform around the support substructure, thereby producing a grating. The device of '663 has more recently become known as the conformal GEMS device, with GEMS standing for Grating ElectroMechanical System. The conformal GEMS device possesses a number of attractive features. It provides high-speed digital light modulation with high contrast and good efficiency. In addition, in a linear array of conformal GEMS devices, the active region is relatively large and the grating period is oriented perpendicular to the array direction. This orientation of the grating period causes diffracted light beams to separate in close proximity to the linear array and to remain spatially separated throughout most of an optical system. When used with laser sources, GEMS devices provide excellent brightness, speed, and contrast, and are capable of providing higher resolution than is available using area, or two-dimensional, spatial light modulators. An example display system using GEMS modulation is disclosed in U.S. Pat. No. 6,411,425, issued Jun. 25, 2002, titled “Electromechanical Grating Display System With Spatially Separated Light Beams,” by Kowarz et al.
With the advent of lower cost laser devices, there is considerable interest in using lasers in display and printing applications. As just a few among many examples: U.S. Pat. No. 6,128,131, issued Oct. 3, 2000, titled “Scaleable Tiled Flat-Panel Projection Color Display,” by Tang discloses a tiled projection color display using laser sources; U.S. Pat. No. 6,031,561, issued Feb. 29, 2000, titled “Printer System Having A Plurality Of Light Sources Of Different Wavelengths,” by Narayan et al. discloses a printing apparatus using lasers for exposing photosensitive media. Continued developments in low cost semiconductor and solid state lasers can be expected to heighten interest in the use of lasers as light sources for these types of imaging applications as well as for scanning, recording, and other uses.
In spite of some promising developments in laser performance, however, there is recognized to be considerable room for improvement. In display applications, for example, where images are formed using three or more light sources having different wavelengths, there are a number of practical constraints. Lasers having suitable wavelengths for display applications, particularly in blue and green spectral regions, can be expensive or difficult to obtain. In printing applications, different sets of wavelengths are required, based on the sensitometric response characteristics of photosensitive media. Printing applications typically demand much higher resolution and overall uniformity than is needed for display or projection applications.
In response to the need for less costly laser sources capable of producing a broad range of wavelengths, laser arrays using organic materials have been developed. U.S. Pat. No. 6,111,902, issued Aug. 29, 2000, titled “Organic Semiconductor Laser,” by Kozlov et al.; U.S. Pat. No. 6,160,828, issued Dec. 12, 2000, titled “Organic Vertical-Cavity Surface-Emitting Laser,” by Kozlov et al.; U.S. Pat. No. 6,396,860, issued May 28, 2002, titled “Organic Semiconductor Laser,” by Kozlov et al., and U.S. Pat. No. 6,330,262, issued Dec. 11, 2001, titled “Organic Semiconductor Lasers,” by Burrows et al. disclose types of Vertical Cavity Surface Emitting Lasers (VCSELs) using organic materials. Copending U.S. patent application Ser. No. 09/832,759 filed Apr. 11, 2001, titled, “Incoherent Light-Emitting Device Apparatus For Driving Vertical Laser Cavity,” by Kahen et al. and Copending U.S. patent application Ser. No. 10/066,829 filed Feb. 4, 2002, titled, “Organic Vertical Cavity Phase-Locked Laser Array Device,” by Kahen also disclose VCSELs having organic-based gain materials with emission in the visible wavelength range. Among advantages of organic-based lasers are lower cost, since the gain material is typically amorphous when compared to gain materials that require a high degree of crystallinity (either inorganic or organic materials). Additionally, lasers based upon organic amorphous gain materials can be fabricated over large areas, without the requirement to produce large regions of single crystalline material; as a result, organic VCSEL arrays can be scaled to arbitrary size. Because of their amorphous nature, organic VCSEL arrays can be fabricated on a wide variety of inexpensive substrates; such as glass, flexible plastics, and silicon and can be more readily tested than conventional semiconductor lasers. Significantly, organic VCSEL arrays are capable of emission over the entire visible range. Optical pumping can be accomplished using low-cost incoherent light sources that are readily available, such as LEDs.
A number of organic VCSEL array characteristics pose problems for use in imaging applications, particularly where a linear spatial light modulator is used. For example, practical, high-power organic VCSEL arrays have aspect ratios that are generally more rectangular than linear. Thus, where higher levels of optical flux are needed, aspherical illumination optics may be required in order to properly shape the illumination beam for a linear spatial light modulator.
A more significant problem relates to the spatial characteristics of the emitted beam from a VCSEL array. Output beam characteristics depend, in large part, on which of two configurations is used. Referring to FIG. 1a, the first configuration, termed an “out-of-phase configuration,” is shown. In the plan view of FIG. 1a, a representative portion of a VCSEL array 100 is shown, comprising an arrangement of individual VCSEL emissive elements 102 and 103. In the out-of-phase configuration, alternate VCSEL emissive elements 102 have one phase; their neighboring VCSEL emissive elements 103, shown shaded, have the opposite phase. For comparison, FIG. 1b shows the alternate “phase-locked” configuration for VCSEL array 100. In this phase-locked configuration, each VCSEL cell 102 has the same phase. The VCSEL emissive elements 102 and 103 in FIGS. 1a and 1b are positioned within the VCSEL array 100 so that the axes of symmetry are the horizontal and vertical axes. The axes of symmetry can be any axes in practice.
Referring to FIG. 2a, there is shown a spatial arrangement of the emitted beam with VCSEL array 100 in the out-of-phase configuration of FIG. 1a. Here, instead of providing a single beam, as would be preferred for ease of handling by optical modulation components, VCSEL array 100 emits four first-order lobes 110a, 110b, 110c, and 110d. Lobes 110a–110d have an unequal height-to-width aspect ratio, as approximated in FIG. 2a; the height of each lobe 110a–110d approximates the corresponding length L of VCSEL array 100, as shown in FIG. 1a. A pair of coordinates is assigned to each lobe 110a–110d for reference. Additional, higher-order lobes are also emitted; however, these higher-order lobes contain only a small portion of emitted light and, for a first approximation, can be ignored. At some distance d, near VCSEL array 100, lobes 110a and 110c overlap, as indicated by a shaded overlap area 112a. Referring to FIG. 2b, there is shown the spatial arrangement for the same beam represented in FIG. 2a, but at a distance of 2d from VCSEL array 100. Here, lobes 110a–110d have spread farther apart, do not overlap, and have a slightly more rounded aspect ratio. One can readily recognize that this distribution of emitted light into lobes 110a–110d requires customized beam-shaping optics within the image modulation mechanism of an imaging apparatus.
In contrast to the lobe arrangement of the out-of-phase configuration, the phase-locked configuration provides a more conventional laser beam. Referring to FIG. 3a, a central lobe 110e now contains a relatively high percentage of the emitted light, with additional light among first order lobes 110a–110d and a very small amount of light in higher order lobes (not shown). At a short distance from VCSEL array 100, central lobe 110e overlaps both first order lobes 110a and 110c at overlap area 112a and overlaps first-order lobes 110a and 110c at overlap areas 112b and 112c, respectively. At about twice the distance from VCSEL array 100, as shown in FIG. 3b, overlap areas 112b and 112c diminish in size and may disappear.
Thus, although VCSEL arrays offer some promise as light sources for modulation by electromechanical grating devices, it is recognized that sizable obstacles remain. As is noted above, the range of aspect ratios over which VCSEL arrays provide high power typically differs from the aspect ratio required for illumination of an electromechanical grating device, making some degree of tradeoff necessary. More significantly, spatial characteristics of the modulated light beam can be relatively complex and can vary, depending on whether an out-of-phase or a phase-locked mode of operation is employed. These differences distinguish VCSEL laser arrays from conventional semiconductor laser sources, therefore, there is a need for a solution that addresses the aspect ratio and spatial content of illumination beams emitted from VCSEL laser arrays operating in either an out-of-phase or a phase-locked mode.